Coordinated Multi-Point (CoMP) transmission/reception has been proposed as a promising technology to meet the 3GPP (Third Generation Partnership Project) LTE-Advanced (LTE-A) requirements by improving performance of cell-edge UEs in particular. In CoMP operation, multiple transmission/reception points (typically geographically separated, but could also be co-located) cooperatively transmit to or receive from one or more users' equipment (UEs) to improve performance, especially the performance of cell-edge UEs. In the case of downlink CoMP, each transmission point, which can have one or more transmit antennas, is a radio unit whose signal covers a geographical area. In general, CoMP techniques refer to a broad range of coordination mechanisms including interference avoidance. CoMP can be used to improve the throughput for cell edge UEs as well as the cell average throughput.
In LTE CRSs may be used by UEs to measure properties of the radio channel with respect to such CSI parameters as a Channel Quality Indicator, CQI. CSI reference signals (CSI-RS) may be also used by terminals to acquire channel-state information. CSI-RS have a significantly lower time/frequency density, thus implying less overhead, compared to the CRS. In CoMP systems the channel measurement for CSI feedback are based on CSI-RS.
For purpose of CSI feedback, interference can be measured on CRS after subtracting channel information from received signals or directly on the channel state information interference measurement resources (CSI-IM) indicated by the network. In CoMP systems the interference measurements for CSI are based on CSI-IM due to its flexibility in supporting measurement for different interference scenarios. However the interference measurement using CSI-IM in some cases may be less accurate than interference measurements cell-specific reference signal (CRS). The less accurate interference measurements on CSI-IM results from the usage of the resource elements (REs) of zero-power channel state information reference signal (ZP CSI-RS) which in case of overlap with another CSI-IM configured on another cell may not capture interference from some of the transmission points.
In contrast, CRS based measurements in the same cases may include interference contribution from the transmitting points even when CRS overlap. This indicates that for high loading scenario, CRS based interference measurements may be more accurate than interference measurements on CSI-IM primary due to smaller number of CSI-IM configurations than CRS sequences.
For CSI-IM interference measurements network indicates to a UE which resource elements (i.e. subcarrier and symbols) the UE is to use to perform interference measurements. The serving cell (node) may not transmit any data on a given resource elements to remove own-cell interference, which can be achieved by configuring ZP CSI-RSs on the same resource elements. The other nodes will transmit data on specified resource elements. Thus, the UE measures interference from the other nodes, e.g., coordinating or neighboring cells. A similar CSI-IM measurements and configurations may be applied in the neighboring nodes. However, due to the limited number of configurations available, collisions between CSI-IM of different nodes will occur. Thus interference from some nodes will not be estimated because of the collisions. This leads to under estimation of interference for CSI feedback.